Brushless DC motor and method of manufacturing brushless DC motor

ABSTRACT

A brushless DC motor including a stator having plural slots; and a rotor which has plural permanent magnets and is divided into three rotor blocks in a rotation axis direction, the three rotor blocks being layered so that the arrangement angles of the rotor blocks differ from each other by an amount of a mechanical angle in a rotary direction that is equivalent to one third of a pulsation period of cogging torque generated by the rotor and stator. A brushless DC motor including a rotor having plural magnetic poles provided at an equal pitch in a circumferential direction by mounting permanent magnets in magnet mounting holes; and a stator having plural slots arranged at an equal pitch in a circumferential direction. The magnetic poles of the rotor include magnetic poles whose magnet deviation angle formed by the central line of an effective polar opening angle and the central line of the magnet mounting hole is the first angle; and magnetic poles whose magnet deviation angle is the second angle different from the first angle.

BACKGROUND OF THE INVENTION

The present invention relates to a brushless DC motor comprising a rotorhaving permanent magnets, and more particularly relates to a brushlessDC motor capable of reducing cogging torque and a manufacturing methodof the same.

A brushless DC motor is a motor that comprises a rotor having permanentmagnets and rotates the rotor by controlling an electric commutatorcircuit for generating a rotational magnetic field in a stator, based ona detection signal representing the rotational position of the rotor.Since the brushless DC motor does not generate mechanical and electricalnoises and has high rotary performance and a long life, it is mainlyused in the cylinder of a VTR, the capstan of a cassette tape deck, aflexible disk driver, a CD player, etc. In recent years, the brushlessDC motor is used in the drive motor of a power steering apparatus forvehicle.

In the brushless DC motor, torque pulsation, i.e., cogging torque, isunavoidably produced because of the presence of slots for winding in thestator and the presence of permanent magnets in the rotor. The coggingtorque is a periodical torque change that is caused in a motor by achange of magnetic flux owing to the position of the rotor.

Conventionally, as a method for preventing the cogging torque of abrushless DC motor, there has been a proposed method for reducingcogging torque by dividing the rotor into two blocks and combining thetwo blocks while displacing the arrangement angle of the rotor blocks ina circumferential direction so that cogging torques generated in the twoblocks are mutually in antiphase with respect to the rotation of therotor.

FIG. 1 is an explanatory view showing an anti-cogging measure taken bysuch a rotor, and shows a perspective view of a rotor 105 as an assemblyof an upper-stage rotor block 110 and a lower-stage rotor block 120. Therotor block 110 comprises an internal rotor core 112, and four permanentmagnets 111 attached to the outer circumference of the rotor core 112 atequal intervals. The rotor block 120 comprises an internal rotor core122, and four permanent magnets 121 attached to the outer circumferenceof the rotor core 122 at equal intervals.

The rotor blocks 110 and 120 are of the same constructions and combinedin an axial direction while displacing the arrangement angles by anamount of a mechanical angle θ12 at which they are mutually in antiphasewith respect to a pulsation period of cogging torque generated by therelationship with an opposing stator. Accordingly, the pulsationcomponents of cogging torques generated in the rotor blocks 110 and 120cancel each other out, thereby reducing the cogging torque of thebrushless DC motor.

FIG. 2 is a view showing the relationship between the conventional rotorblocks 110, 120 and the stator as a cross section in a directionperpendicular to the rotary shaft. A stator 101 is formed by layering anumber of thin electromagnetic steel plates and fixing them integrally,and comprises a yoke 102 as an outer circumferential portion and teeth103 that are provided at equal intervals to protrude from the yoke 102toward the center. Adjacent teeth 103 form a slot 104 together with theyoke 102. Actually, armature windings are wound on the teeth 103 andstored in the slots 104.

FIGS. 3A and 3B are waveform charts for explaining the above-mentionedanti-cogging torque measure. The vertical axis indicates cogging torque,while the horizontal axis shows the rotation angle of the rotor 105.Each of cogging torque Tc1 generated in the upper-stage rotor block 110and cogging torque Tc2 generated in the lower-stage rotor block 120 hasa pulsation period θ11.

In the case where the cogging torques Tc1 and Tc2 are such sinusoidalwaveforms that have the same change in an increasing direction and adecreasing direction with respect to the center of amplitude, if therotor blocks 110 and 120 are combined to have a phase shiftcorresponding to a half period θ12 of the pulsation period θ11, thepulsation components of the cogging torques Tc1 and Tc2 of the rotorblocks 110 and 120 cancel each other out and, ideally, their compositecogging torque Tct is made a straight waveform having no pulsation asshown in FIG. 3B.

A prerequisite for effectively realizing the above-mentioned method isthat the pulsation waveforms of the cogging torques Tc1 and Tc2generated in the respective rotor blocks 110 and 120 are mutually inantiphase in the moving direction of the rotor with respect to thecenter of the amplitude and have magnitudes so that the cogging torquesTc1 and Tc2 cancel each other out. In an actual brushless DC motor, itis possible to significantly improve the pulsation period of coggingtorque by the above-mentioned method, but there is a problem that smallcogging torque pulsation remains. In order to improve such small coggingtorque and design a brushless DC motor having no distortion in therotational operation, conventionally an abrupt change of cogging torqueis prevented and the pulsation is restricted by widening the gap betweenthe rotor and the stator to a large extent, providing unequal gaps inthe circumferential direction and intentionally leaking a part ofmagnetic flux from the permanent magnets of the rotor near the regionbetween the magnetic poles. However, the motor efficiency is sacrificed.

FIGS. 4A and 4B are waveform charts for explaining the influence of suchan anti-cogging torque measure, in which the same codes as in FIGS. 3Aand 3B are used. The pulsation of the cogging torques Tc1 and Tc2 iscaused by a change of magnetic flux distribution which occurs betweenthe permanent magnets 111, 121 and the stator 101 with respect to therotary direction of the rotor 105. In particular, the presence of theopenings of the slots 104 of the stator largely affects this change.There is a difference in the magnetic flux distribution in the magneticpath of the stator 101 and the rotor 105 between the case where theregion between the magnetic poles of the rotor 105 approaches theopening of the slot 104 and the case where the region between themagnetic poles moves away form the opening. As a result, there is apossibility that the pulsation of cogging torque does not become asinusoidal waveform having the same change in the increasing directionand the decreasing direction with respect to the center of amplitude.

Moreover, in an ordinary motor structure, the opening of the slot 104 ofthe stator 101 is narrowed to ensure an interlinkage magnetic flux fromthe rotor 105 to the stator 101, and the width of the tooth 103 is madelarger than the width of the opening of the slot 104 so as to realizehigh torque and high efficiency. Therefore, the cogging torque has asmall change in a section where the region between magnetic poles of therotor 105 faces the tooth 103 during the rotational movement, but alarge change in a section where the region faces the opening of the slot104. Thus, as shown in FIG. 4A, waveform distortion including evenharmonics symmetrical about a point may be caused. Even if thepulsations of such cogging torque waveforms are combined whiledisplacing them by an amount corresponding to the half period θ12 of thepulsation period 011, there is a problem that the pulsation of coggingtorque as shown in FIG. 4B still remains.

The cogging torque Tct of the rotor obtained by dividing the rotor intotwo blocks and layering the blocks while displacing them by an amountcorresponding to the half period θ12 of the pulsation period θ11 ofcogging torque is shown by expression (1). Here, T0 is a peak value of afundamental wave component of cogging torque when the rotor is notdivided, x is an electrical angle of the angle of an arbitraryrotational position of the rotor, n is a natural number, and kn is theratio of the 2n-th harmonics content to the fundamental wave.$\begin{matrix}\begin{matrix}{{Tct} = {{{TO}/2}\left\{ {{\sin(x)} + {\sum\limits_{n = 1}^{\infty}\quad{{kn} \cdot {\sin\left( {2{nx}} \right)}}} +} \right.}} \\\left. {{\sin\left( {x + \pi} \right)} + {\sum\limits_{n = 1}^{\infty}\quad{{kn} \cdot {\sin\left( {{2{nx}} + {2n\quad\pi}} \right)}}}} \right\} \\{= {{TO}{\sum\limits_{n = 1}^{\infty}\quad{{kn} \cdot {\sin\left( {2{nx}} \right)}}}}}\end{matrix} & (1)\end{matrix}$

It is apparent from the expression (1) that, in the brushless DC motorcomprising the rotor divided into two blocks, the fundamental wavecomponents of cogging torques cancel each other out and are thuseliminated, but there is a problem that the even harmonics componentsremain.

There is another conventional anti-cogging torque measure shown in FIG.5, for example. In FIG. 5, the same parts as those shown in FIG. 2 aredesignated with the same numbers. In the example shown in FIG. 5, onewith an outer circumference having a curvature larger than the curvatureof the outer circumference of the rotor core 106 is used as thepermanent magnet 107 of the rotor 105. Moreover the gap between thepermanent magnet 107 and the teeth 103 of the stator 101 graduallyincreases from the center toward the ends of the permanent magnet 107 inthe circumferential direction. Therefore, when the rotor core 106rotates, the magnetic flux interlinking with the teeth 103 changessmoothly instead of stepwise. Thus, a reduction in cogging torque ismade. Some countermeasure produces a similar effect by changing theshape of the outer circumference of the rotor core 106 instead ofchanging the shape of the permanent magnets 107.

FIG. 6 shows still another conventional anti-cogging torque measure. Inthe example shown in FIG. 6, a skew angle θS is provided for thearrangement of magnetic poles in the axial direction of the rotor 105.Hence, when the rotor 105 rotates, the timing in which the boundarybetween the magnetic poles crosses the teeth of the stator variesaccording to a position in the axial direction of the rotor 105. Thus,the change of the magnetic flux interlinking with the teeth is mademoderate, and the cogging torque is reduced.

However, both of the conventional techniques shown in FIGS. 5 and 6suffer from problems including poor magnetic efficiency. In thetechnique shown in FIG. 5, since the average gap between the permanentmagnets 107 and the teeth 103 is large, the magnetic efficiency is poorand a rotary output proportional to the magnetic force of the permanentmagnets 107 can not be obtained. Moreover, it is necessary to performvarious analysis and trial manufacture to determine the shape of thepermanent magnets 107 or the outer circumference of the rotor core 106,resulting in high development costs. Furthermore, it is necessary toprocess the small configurations accurately, and thus the processingitself is difficult. Nevertheless, an objective to reduce the coggingtorque is not sufficiently achieved. In particular, when strongrare-earth based permanent magnets are used to meet the demand for areduction in size as in recent years, the cogging torque in itself isconsiderably large. Therefore, such a method is not sufficient.

Similarly, the technique shown in FIG. 6 suffers from poor magneticefficiency and can not obtain a sufficient rotary output. The reason forthis is that there is the skew angle θS in the arrangement of magneticpoles and consequently the effective magnetic flux of the magnetic polesbecomes smaller by a corresponding amount. In the example shown in FIG.6, one magnetic pole occupies substantially a parallelogram region onthe side face of the rotor 105. In a portion near the acute apex, themagnetic flux in the portion does not effectively perform the functionof the motor. Therefore, like the example shown in FIG. 5, thistechnique can not obtain a sufficient rotary output.

In recent years, the brushless DC motor is often made to have a smallsize and high output by using a rare-earth material, etc. for thepermanent magnets, and tends to be used as a magnetic circuit in a highmagnetic flux density region of the thin electromagnetic steel plates.On the other hand, there is a problem that the motor performance isdegraded as a result of the promotion of the reduction in the size ofthe motor and the generation of extremely high heat by the motor for thesize of the motor. In order to solve this problem, notch portions areprovided in the outer circumference of the stator and a cool air or thelike is caused to flow through the notch portions to cool the motor andlimit the generation of heat. Besides, in order to achieve anotherobjective to ensure a punching yield of electromagnetic steel plates anda gap in the layering direction for sticking means such as welding,notch portions are provided on the outer circumference side of thestator.

FIG. 7 is a perspective view showing an example of the stator of aconventional brushless DC motor having such notch portions. In FIG. 7,the same parts as in those of FIGS. 2 and 5 are designated with the samenumbers.

A notch portion 109 running from the upper end to the lower end of thestator 101 is provided on the outer circumferential surface of the yoke102, at a position near the outside of every third tooth 103. The notchportions 109 are provided on the outer circumference as the coolingpaths for releasing heat during the operation of the motor and for thepurpose of easing the welding that is performed for fixing plurallayered steel plates (by using the protrusions in the notch portions109) and easing the punching of material to improve the yield. Byproviding the notch portions 109 on the outer circumference, it ispossible to prevent the welded section from fixing out of the outercircumference of the stator 101 in welding the thin electromagnetic thinplates to fix them integrally. Moreover, the notch portions 109 areoften provided for the purpose of saving the material of the thinelectromagnetic steel plates of the stator 101. As described above, eachof the notch portions 109 runs from the upper end to the lower end ofthe stator 101 and has a length S0 in the layering direction.

In the above-described conventional stator 101, since the notch portions109 are aligned with the layering direction, there is a difference inthe magnetic resistance seen from the inside of the stator 101 between aregion of the teeth 103 where the notch portion 109 is present on theouter circumference side of the stator 101 and a region where the notchportion 109 is not present. In the case where a rotor having permanentmagnets is positioned inside the stator 101, a magnetic circuit in whichthe magnetic flux flows is formed between the stator 101 and the rotorwhich faces the teeth 103 and have permanent magnets arranged so thatadjacent permanent magnetic have opposite polarities. This magneticcircuit is formed as a magnetic closed circuit composed mainly of theshortest path between adjacent opposite poles. The shortest magneticcircuit starting from a region between the magnetic poles of thepermanent magnets of the rotor is most of the causes of generation ofcogging torque. In this magnetic circuit, there is a big difference inthe magnetic flux amount between a region of the teeth 103 where thenotch portion 109 is present on the outer circumference side of thestator 101 and a region where the notch portion 109 is not present.Thus, the difference in the magnetic flux amount according to thepositions in the rotary direction of the rotor is one of the causes ofcogging torque, and is a cause of generation of sound and vibration.

FIG. 8 is a view showing the state of magnetic flux in such a brushlessDC motor. Here, the rotor 105 having the permanent magnets 107 attachedto the surface of the rotor core 106 is disposed inside the stator 101shown in FIG. 7. The stator 101 is formed by layering necessary piecesof thin electromagnetic steel plates having a portion equivalent to thenotch portion 109 on the outer circumference side of every third portionequivalent to the tooth 103. Note that the rotor 105 may be aburied-type rotor having permanent magnets buried in the rotor core 106.

A magnetic flux generated by the relative positional relationshipbetween the stator 101 and the regions between the magnetic poles of theopposing permanent magnets 107 of the rotor 105 flows in respectiveportions of the stator 101. The magnetic flux amount in a magnetic patha in a region of the teeth 103 where a notch portion 109 is present onthe outer circumference side of the stator 101 is denoted as φ1, themagnetic flux amount in a magnetic path b in a region of the teeth 103where no notch portion 109 is present on the outer circumference side ofthe stator 101 is denoted as φ2, and the magnetic flux amount in amagnetic path c in a region where a notch portion 109 different fromthat for the flux amount φ1 is present is denoted as φ3. Here, if thenotch portions 109 have the same configuration, it is clear that onlythe difference between the magnetic flux amounts φ1 and φ3 is theposition of the notch portion 109 in the magnetic path, and the magneticflux amounts φ1 and φ3 are the same in magnitude.

Here, as shown in FIG. 8, when straight lines A, B and C are drawn fromthe center of the shaft hole of the rotor 105 through the center of theslots 104 toward the outer circumference of the stator 101, if a regionbetween the magnetic poles of the permanent magnets 107 of the rotor 105is positioned on the straight line A, the magnetic flux from thepermanent magnets 107 near the region between the magnetic poles forms aclosed circuit of the magnetic flux amount φ1 by the magnetic path ashown by a dotted line. Besides, when the rotor 105 rotates clockwiseand the region between the magnetic poles of the permanent magnets 107reaches the straight line B, the magnetic flux from the permanentmagnets 107 near the region between the magnetic poles forms a closedcircuit of the flux amount φ2 by the magnetic path b shown by a dottedline. When the rotor 105 further rotates clockwise and the regionbetween the magnetic poles of the permanent magnets 107 reaches thestraight line C, the magnetic flux from the permanent magnets 107 nearthe region between the magnetic poles forms a closed circuit of the fluxamount φ3 by the magnetic path c shown by a dotted line.

There is a difference in the cross sectional area of the magnetic pathdue to the presence and absence of the notch portion 109 in the magneticpath, between the state where the region between the magnetic poles ofthe permanent magnets 107 of the rotor 105 is positioned on the straightline A and the state where the region between the magnetic poles ispositioned on the straight line B. Accordingly, there is a difference inthe magnetic resistance, and the flux amounts are φ1<φ2. Similarly,there is a difference in the cross sectional area of the magnetic pathdue to the presence and absence of the notch portion 109, between thestate where the region between the magnetic poles of the permanentmagnets 107 of the rotor 105 is positioned on the straight line B andthe state where the region between the magnetic poles is positioned onthe straight line C. Accordingly, there is a difference in the magneticresistance, and the flux amounts are φ3<φ2.

Hence, when the region between the magnetic poles of the permanentmagnets 107 of the rotor 105 is positioned on the straight line B havingno notch portion 109 on the outer circumference side of the stator 101,the strongest magnetic coupling is obtained between the rotor 105 andthe stator 101. The change in cogging torque resulting from suchphenomena is that the largest cogging torque appears when the regionbetween the magnetic poles approaches or moves away from the position ofthe straight line B because the magnetic coupling is strong in thatposition as shown in FIG. 9 and described above. In FIG. 9, the verticalaxis indicates the cogging torque TC and the horizontal axis shows therotation angle θ of the rotor 105, and the positions of the straightlines A to C shown in FIG. 8 correspond to the positions of the straightlines A to C of FIG. 9.

However, in the brushless DC motor, since the notch portions 109 areprovided on the outer circumference side of the stator 101, the size ofthe cross sectional area of the magnetic paths varies because of thedifference in the magnetic paths as described above. As a result, thebrushless DC motor suffers from a problem of deterioration of thepulsation of cogging torque.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a brushless DC motorcapable of effectively eliminating cogging torque.

It is another object of the present invention to provide a brushless DCmotor capable of certainly reducing cogging torque without making almostno sacrifice to the output characteristics.

It is still another object of the present invention to provide abrushless DC motor having notch portions or cavity portions in itsstator and capable of reducing the pulsation of cogging torque.

It is yet another object of the present invention to provide a method ofmanufacturing a brushless DC motor, capable of manufacturing a brushlessDC motor of excellent quality.

A brushless DC motor according to the first aspect is a brushless DCmotor comprising: a rotor having plural permanent magnets; and a statorhaving plural slots, wherein the rotor is divided into three rotorblocks in a rotation axis direction, and the three rotor blocks arelayered so that arrangement angles of the rotor blocks differ from eachother by an amount of a mechanical angle in a rotary direction that isequivalent to one third of a pulsation period of cogging torquegenerated by the rotor and stator. It is thus possible to realize abrushless DC motor capable of effectively eliminating cogging torque.

A brushless DC motor according to the second aspect is based on thefirst aspect, wherein a sum of an effective polar opening angle of onepermanent magnet and a difference between the arrangement angles of therotor block located on one end and the rotor block located on the otherend is not more than a pole pitch angle of the rotor. Thus, eachpermanent magnet does not overlap adjacent different magnetic pole, andevery magnetic flux from the permanent magnet becomes an effectivemagnetic pole. Therefore, in comparison with a rotor that uses skewmagnets and a rotor that uses a ring magnet magnetized on the skew asanti-cogging measures which are usually performed in this type of motor,the amount of permanent magnets to be used can be decreased and thecogging torque can be reduced without deteriorating the electricalcharacteristics. Moreover, it is possible to perform built-inmagnetization for magnetizing the rotor in a non-magnetic state by usingthe stator as a magnetic yoke after assembling the motor. Accordingly,it is possible to eliminate a difficult handling work caused by therotor blocks being attracted to the stator, case, etc. and to preventdust such as an iron powder attracted to the rotor blocks from beingcarried into the case, thereby realizing a brushless DC motor havinggood quality.

A brushless DC motor according to the third aspect is a brushless DCmotor comprising: a rotor having plural permanent magnets; and a statorhaving plural slots, wherein each of the permanent magnets is dividedinto three permanent magnets in a rotation axis direction, and the threepermanent magnets are layered so that arrangement angles of thepermanent magnets differ from each other by an amount of a mechanicalangle in a rotary direction that is equivalent to one third of apulsation period of cogging torque generated by the rotor and stator. Itis therefore possible to realize a brushless DC motor capable ofeffectively eliminating cogging torque. Moreover, since this is achievedonly by the arrangement of the permanent magnets without dividing therotor core itself, it is possible to realize a brushless DC motor thatis easy to assemble.

A brushless DC motor according to the fourth aspect is based on thethird aspect, wherein a sum of an effective polar opening angle of oneof the permanent magnets and a difference between the arrangement anglesof the permanent magnets located on both ends in the rotation axisdirection among the three permanent magnets is not more than a polepitch angle of the rotor. Accordingly, each permanent magnet does notoverlap adjacent different magnetic pole, and every magnetic flux fromthe permanent magnets becomes an effective magnetic pole. Therefore, incomparison with a rotor that uses skew magnets and a rotor that uses aring magnet magnetized on the skew as anti-cogging measures which areusually performed in this type of motor, the amount of permanent magnetsto be used can be decreased and the cogging torque can be reducedwithout deteriorating the electrical characteristics. Moreover, sincethis is achieved only by the arrangement of the permanent magnetswithout dividing the rotor core itself, it is possible to realize abrushless DC motor that is easy to assemble.

A brushless DC motor according to the fifth aspect is a brushless DCmotor comprising: a rotor having plural magnetic poles provided at anequal pitch in a circumferential direction by mounting permanent magnetsin magnet mounting holes; and a stator having plural slots arranged atan equal pitch in a circumferential direction, wherein the magneticpoles of the rotor include magnetic poles whose magnet deviation angleformed by a central line of an effective polar opening angle and acentral line of the magnet mounting hole is a first angle; and magneticpoles whose magnet deviation angle is a second angle. Of course, thefirst angle and the second angle are different angles. In this brushlessDC motor, the magnetic poles of the rotor include magnetic poles whosemagnet deviation angle is the first angle (hereinafter referred to asthe “magnetic poles of the first angle”) and magnetic poles whose magnetdeviation angle is the second angle (hereinafter referred to as the“magnetic poles of the second angle”). Therefore, the phase of coggingtorque generated by the magnetic pole of the first angle and the phaseof cogging generated by the magnetic pole of the second angle do notcoincide with each other. In other words, these cogging torques do notreach the peaks simultaneously because of the following reason. In thesemagnetic poles, there is a difference in the timing in which an end ofthe magnet crosses an end of the slot of the stator by rotation. Forthis reason, the overall cogging torque of the brushless DC motor isreduced as compared to a construction where all the magnetic poles havean equal magnet deviation angle.

A brushless DC motor according to the sixth aspect is based on the fifthaspect, wherein a difference θ6 between the second angle and the firstangle is within a range defined by b 0.2×θ7≦θ6≦θ5−(0.2×θ7), where θ5 isa slot pitch angle of the stator, and θ7 is a slot opening angle of thestator. In this case, if the magnetic pole of the first angle is set asa standard, then, in the magnetic pole of the second angle, thepermanent magnet is mounted at a position preceding (or succeeding) byan amount of the angle θ6 in the rotary direction. Here, if the firstangle is zero, then the second angle is θ6. In this case, in themagnetic pole of the first angle, the central line of the effectivepolar opening angle and the central line of the magnet mounting holecoincide with each other. On the other hand, in the magnetic pole of thesecond angle, the central line of the effective polar opening angledeviates from the central line of the magnet mounting hole by an amountof θ6. Hence, there is a corresponding difference in the timing in whichthe cogging torque reaches the peak between these magnetic poles. Ifthis difference is too small, the effect of reducing the overall coggingtorque of the brushless DC motor is not sufficient. On the other hand,if the difference is too large, the timings in which the cogging torquesreach the peaks become close to each other because of the relationshipwith adjacent slot of the stator. More specifically, when the magnetdeviation angle is as large as the slot pitch angle θ5 of the stator,such a result is given. When the difference between the first angle andthe second angle is within the above-mentioned range, such a result isnot given and the overall cogging torque of the brushless DC motor iscertainly restricted.

A brushless DC motor according to the seventh aspect is based on thefifth or sixth aspect, wherein the number of the magnetic poles of thefirst angle and the number of the magnetic poles of the second angle areequal to each other on the rotor. It is therefore possible to morecertainly restrict the overall cogging torque by the mutual cancellationof the cogging torque waveforms of the magnetic poles of the first angleand the magnetic poles of the second angle.

A brushless DC motor according to the eighth aspect is based on any oneof the fifth through seventh aspects, wherein the magnetic pole of thefirst angle and the magnetic pole of the second angle are arranged nextto each other on the rotor. Since the rotor is a rotary member, therotary balance must be taken into consideration. Changing the magnetdeviation angle by the magnetic poles may shift the center of gravity ofthe rotor from the center of the axis and deteriorate the rotarybalance. However, it is possible to minimize the deterioration of therotary balance by arranging the magnetic pole of the first angle and themagnetic pole of the second angle next to each other.

A brushless DC motor according to the ninth aspect is based on any oneof the fifth through seventh aspects, wherein the rotor is divided intoplural bocks in a rotation axis direction, and the magnetic pole of thefirst angle and the magnetic pole of the second angle are arranged atcorresponding positions in the rotation axis direction in differentblocks. The corresponding positions in the rotation axis direction meanthe positions having the same angular coordinates about the axis.Accordingly, in the rotor as a whole, the cancellation of the coggingtorque waveforms is achieved within a single magnetic pole. It istherefore possible to obtain such an effect that as if the coggingtorque generated by a single magnetic pole is reduced. Consequently, theoverall cogging torque of the brushless DC motor is effectivelyrestricted.

A brushless DC motor according to the tenth aspect is based on the sixthaspect, wherein the rotor further includes magnetic poles whose magnetdeviation angle is a third angle (hereinafter referred to as the“magnetic poles of the third angle”), a difference θ6 between the secondangle and the first angle is within the range defined by the aboveexpression, and a difference between the third angle and the first anglehas the same value as and opposite sign to θ6, i.e., −θ06. In this case,if the magnetic pole of the first angle is set as a standard, then, inthe magnetic pole of the second angle, the permanent magnet is mountedat a position preceding only by an amount of the angle θ6 in the rotarydirection. In the magnetic pole of the third angle, the permanent magnetis mounted at a position succeeding only by an amount of the angle θ6 inthe rotary direction. It is also possible to switch the preceding andsucceeding relation. Accordingly, the cancellation of cogging torques isperformed by three waveforms mutually shifted at equal intervals.Therefore, the overall cogging torque of the brushless DC motor can bemore certainly restricted.

A brushless DC motor according to the eleventh aspect is based on thetenth aspect, wherein the number of the magnetic poles of the firstangle, the number of the magnetic poles of the second angle and thenumber of the magnetic poles of the third angle are equal to each other.Therefore, the overall cogging torque of the brushless DC motor can bemore certainly restricted by the cancellation of the cogging torquewaveforms among the magnetic poles of the first angle, the magneticpoles of the second angle and the magnetic poles of the third angle.

A brushless DC motor according to the twelfth aspect is based on theeleventh aspect, wherein a total number of the magnetic poles of therotor is an integral multiple of b 6, and all of the magnetic poles ofthe rotor are any magnetic pole among the magnetic poles of the firstangle, the magnetic poles of the second angle and the magnetic poles ofthe third angle. In this construction, since there is no extra waveformcomponents, it is possible to more certainly restrict the coggingtorque. Note that, in a construction where the rotor is divided intoplural blocks in the axial direction, the total number of magnetic polesis the product of the number of blocks and the number of the magneticpoles in each block.

A brushless DC motor according to the thirteenth aspect is a brushlessDC motor comprising: a rotor having plural magnetic poles provided at anequal pitch in a circumferential direction by mounting permanent magnetsin magnet mounting holes; and a stator having plural slots arranged atan equal pitch in a circumferential direction, wherein the rotorcomprises convex portions corresponding to the magnetic poles on itscircumference, and the magnetic poles of the rotor include magneticpoles whose convex portion deviation angle formed by a central line ofthe convex portion and a central line of the magnet mounting hole is afirst angle; and magnetic poles whose convex portion deviation angle isa second angle. In this brushless DC motor, the magnetic poles of therotor include the magnetic poles whose convex portion deviation angle isthe first angle and the magnetic poles whose convex portion deviationangle is the second angle. The phases of cogging torques generated bythese magnetic poles do not coincide with each other. More specifically,these cogging torques do not reach the peaks simultaneously because ofthe following reason. In these magnetic poles, there is a difference inthe timing in which an end of the convex portion crosses an end of theslot of the stator by rotation. Thus, the overall cogging torque of thebrushless DC motor is reduced as compared to a construction where allthe magnetic poles have an equal convex portion deviation angle.Moreover, since the convex portions of the rotor is very light, theyhave almost no influence on the position of the center of gravity of therotor.

A brushless DC motor according to the fourteenth aspect is a brushlessDC motor comprising: a rotor having plural magnetic poles provided at anequal pitch in a circumferential direction by mounting permanent magnetsin magnet mounting holes; and a stator having plural slots arranged atan equal pitch in a circumferential direction, wherein the rotorcomprises convex portions corresponding to the magnetic poles on itscircumference, and the magnetic poles of the rotor include magneticpoles whose magnet deviation angle and convex portion deviation angleare both first angle; and magnetic poles whose magnet deviation angleand convex portion deviation angle are both second angle. In thisbrushless DC motor, in the magnetic pole where the magnet deviationangle and convex portion deviation angle are both first angle, thecentral line of the effective polar opening angle and the central lineof the convex portion coincide with each other. Similarly, they coincidewith each other in the magnetic pole where the magnet deviation angleand convex portion deviation angle are both second angle. Therefore, themagnetic force is efficiently utilized. Of course, the cogging torquereducing effect by the difference in the magnet deviation angle andconvex portion deviation angle between the magnetic poles is alsoobtained.

A brushless DC motor according to the fifteenth aspect is a brushless DCmotor comprising notch portions or cavity portions provided near anouter circumference side of a part of teeth of a stator constructed bylayering plural steel plates, wherein the steel plates are layered whiledisplacing the steel plates at a predetermined angle in acircumferential direction so that a length of the notch portions or thecavity portions in a layering direction of each of the teeth of thelayered steel plates is substantially equal. In this brushless DC motor,notch portions or cavity portions are provided near the outercircumference side of a part of the teeth of the stator constructed bylayering plural steel plates, and the steel plates are layered whiledisplacing the steel plates at a predetermined angle in acircumferential direction so that a length of the notch portions or thecavity portions in the layering direction of each tooth is substantiallyequal. Accordingly, since the difference in the size of the crosssectional area of the magnetic paths due to the notch portions or cavityportions can be made smaller, it is possible to realize a brushless DCmotor comprising a stator having notch portions or cavity portions andcapable of reducing the pulsation of cogging torque.

A brushless DC motor according to the sixteenth aspect is based on thefifteenth aspect, wherein a substantially equal number of the steelplates are layered at an equal angle to form blocks, and the steelplates are layered while displacing the blocks at a predetermined anglein a circumferential direction. In this brushless DC motor, the statorcan be formed by layering blocks having aligned notch portions or cavityportions, thereby realizing a brushless DC motor comprising a statorhaving notch portions or cavity portions and capable of reducing thepulsation of cogging torque.

A brushless DC motor according to the seventeenth aspect is based on thefifteenth or sixteenth aspect, wherein the notch portions or cavityportions are formed in the steel plates for every other tooth. In thisbrushless DC motor, the difference in the size of the cross sectionalarea of the magnetic paths due to the notch portions or cavity portionscan be made smaller and equalized, thereby realizing a brushless DCmotor comprising a stator having an appropriate number of notch portionsand cavity portions and capable of reducing the pulsation of coggingtorque.

A brushless DC motor according to the eighteenth aspect is based on anyone of the fifteenth through seventeenth aspects, wherein the notchportions or cavity portions are arranged so that adjacent notch portionsor cavity portions of the angularly displaced steel plates in a crosssectional view in the layering direction are in point contact with orseparated from each other. In this brushless DC motor, the difference inthe size of the cross sectional area of the magnetic paths due to thenotch portions or cavity portions can be made smaller and equalized,thereby realizing a brushless DC motor comprising a stator having notchportions or cavity portions with less magnetic flux leakage and capableof reducing the pulsation of cogging torque.

A method of manufacturing a brushless DC motor according to thenineteenth aspect is a method of manufacturing a brushless DC motor ofany one of the first through fourth aspects, wherein the permanentmagnets of the rotor are produced by magnetizing the rotor blocks orrotor by using the stator as a magnetic yoke after assembling the motor.Accordingly, it is possible to eliminate a difficult handling workcaused by the permanent magnets being attracted to the stator, case,etc. and prevent dust such as an iron powder attracted to permanentmagnets from being carried into the case, thereby enabling themanufacturing of a brushless DC motor having good quality.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a view showing one example of anti-cogging torque measuretaken by a conventional brushless DC motor;

FIG. 2 is a view showing the relationship between the conventional rotorblocks and the stator;

FIGS. 3A and 3B are waveform charts for explaining the conventionalanti-cogging torque measure;

FIGS. 4A and 4B are waveform charts for explaining the influence of theconventional anti-cogging torque measure;

FIG. 5 is a view showing another example of anti-cogging torque measuretaken by the conventional brushless DC motor;

FIG. 6 is a view showing still another example of anti-cogging torquemeasure taken by the conventional brushless DC motor;

FIG. 7 is a perspective view showing an example of the stator of theconventional brushless DC motor;

FIG. 8 is a view for explaining the state of magnetic flux in theconventional brushless DC motor;

FIG. 9 is a waveform chart showing the change of cogging torque of theconventional brushless DC motor;

FIG. 10 is a perspective view showing the construction of the rotor of abrushless DC motor according to the first embodiment of the presentinvention;

FIG. 11 is a view showing the displaced state of the rotor blocks;

FIG. 12A is a waveform chart of cogging torques of three rotor blocks;

FIG. 12B is a waveform chart of the composite cogging torque of FIG.12A;

FIG. 13A is a waveform chart of cogging torques when the cogging torquesof the rotor blocks are being distorted;

FIG. 13B is a waveform chart of the composite cogging torque of FIG.13A;

FIGS. 14A, 14B and 14C are views showing the construction of the rotorof a brushless DC motor according to the second embodiment of thepresent invention;

FIG. 15 is a cross sectional view showing partly the construction of abrushless DC motor according to the third embodiment of the presentinvention;

FIG. 16 is a linearly developed view showing the relationship betweenthe magnetic poles of the rotor and the teeth of the brushless DC motorof FIG. 15;

FIG. 17 is a waveform chart showing the composite condition of coggingtorque of the brushless DC motor of FIG. 15 in comparison with theconventional one;

FIG. 18 is a graph showing the relationship between a deviation angle θ6and the composite cogging torque;

FIG. 19 is a view showing one example of the entire structure of therotor of the brushless DC motor of FIG. 15;

FIGS. 20A and 20B are cross sectional views showing the structure of therotor of a brushless DC motor according to the fourth embodiment of thepresent invention;

FIG. 21 is a cross sectional view showing the structure of the rotor ofa brushless DC motor according to the fifth embodiment of the presentinvention;

FIG. 22 is a waveform chart showing the composite condition of coggingtorques of the brushless DC motor of FIG. 21;

FIGS. 23A and 23B are cross sectional views showing the structure of therotor of a brushless DC motor according to the sixth embodiment of thepresent invention;

FIG. 24 is a cross sectional view showing the structure of the rotor ofa brushless DC motor according to the seventh embodiment of the presentinvention;

FIG. 25 is a cross sectional view showing the structure of the rotor ofa brushless DC motor according to the eighth embodiment of the presentinvention;

FIG. 26 is a cross sectional view showing a modified example of thestructure of the rotor of the brushless DC motor of the presentinvention;

FIG. 27 is a cross sectional view showing another modified example ofthe structure of the rotor of the brushless DC motor of the presentinvention;

FIG. 28 is a perspective view showing the construction of one example ofthe stator of a brushless DC motor according to the ninth embodiment ofthe present invention;

FIG. 29 is a side view showing the construction of the stator;

FIG. 30 is a view showing the state of magnetic flux in the brushless DCmotor of the present invention;

FIG. 31 is a view showing the change of cogging torque of the brushlessDC motor of the present invention;

FIG. 32 is a perspective view showing the construction of anotherexample of the stator of the brushless DC motor according to the ninthembodiment of the present invention; and

FIG. 33 is a view showing the relationship between the pitch of teethand notch portions.

DETAILED DESCRIPTION OF THE INVENTION

The following description will explain the present invention withreference to the drawings illustrating some embodiments thereof.

(First Embodiment)

FIG. 10 is a perspective view showing the construction of a rotor of abrushless DC motor according to the first embodiment of the presentinvention. This rotor 5 is constructed by a rotor block 10 in the upperstage, a rotor block 20 in the middle stage, and a rotor block 30 in thelower stage. The rotor block 10 comprises an internal rotor core 12, andfour permanent magnets 11 attached to the outer circumference of therotor core 12 at equal intervals. The rotor block 20 comprises aninternal rotor core 22, and four permanent magnets 21 attached to theouter circumference of the rotor core 22 at equal intervals. The rotorblock 30 comprises an internal rotor core 32, and four permanent magnets31 attached to the outer circumference of the rotor core 32 at equalintervals.

The rotor cores 12, 22 and 32 have the same size, and each of whichcomprises four coupling member insertion holes 13, 23, 33 for fixing therotor blocks 10, 20 and 30 integrally with coupling members. Thepermanent magnets 11, 21 and 31 have the same size. Note that thepermanent magnets 11, 21 and 31 are stuck to the rotor cores 11, 21 and31, respectively, with an adhesive or the like. Alternatively, althoughit is not shown in the drawing, a protective cover or the like is fixedto the outer circumference of each of the permanent magnets 11, 21 and31 by shrinkage fitting, press fitting or other method. In this case,the material of the protective cover is suitably selected according tothe condition of use of a non-magnetic material or a magnetic material.

The upper-stage rotor block 10 and the middle-stage rotor block 20 arelayered in the axial direction so that they are mutually displaced atangle θ1 in the circumferential direction. The middle-stage rotor block20 and the lower-stage rotor block 30 are layered in the axial directionso that they are mutually displaced at angle θ2 in the oppositedirection to the upper-stage rotor block 10 and the middle-stage rotorblock 20. Thus, the displacement angle between the upper-stage rotorblock 10 and the lower-stage rotor block 30 is the sum of the angle θ1and the angle θ2, and the rotor blocks 10, 20 and 30 are layered whilesequentially displacing them in one direction by rotation. The mutualdisplacement angles θ1 and θ2 of the rotor blocks 10, 20 and 30 arecorresponding to an electrical angle of 120° of a pulsation period ofcogging torque (one third of the pulsation period) generated in therotor which is not divided.

FIG. 11 is a view showing the displaced state of each of the rotorblocks 10, 20 and 30 by the plan views of the respective rotor blocks10, 20 and 30. The same parts as in FIG. 10 are designated with the samenumbers. A common central straight line K1 is drawn through the rotorblocks 10, 20 and 30 to show the displacement angles of the respectiveblocks. The rotor block 10 is displaced by counterclockwise rotationonly at the angle θ1 formed by the common central straight line K1 and acentral line J1 of the permanent magnet 11 located on the common centralstraight line K1. The rotor block 30 is displaced by clockwise rotationonly at the angle θ2 formed by the common central straight line K1 and acentral line J2 of the permanent magnet 31 located on the common centralstraight line K1. The rotor block 20 is not displaced, and the centralline of the permanent magnet 21 located on the common central straightline K1 is positioned on the common central straight line K1.

These rotor blocks 10, 20 and 30 are aligned with the common centralline K1 and stuck integrally by inserting caulking pines or bolts, forexample, into the coupling member insertion holes 13, 23 and 33. As aresult, the rotor blocks 10, 20 and 30 are layered so that the magneticpole center positions of the permanent magnets 11, 21 and 31 aredisplaced sequentially in the circumferential direction and thepositions of the regions between the magnetic poles of the permanentmagnets 11, 21 and 31 are relatively displaced by the same angle,thereby constructing the rotor.

If such a rotor rotates in the counterclockwise direction, the pulsationof cogging torque caused by each of the rotor blocks 10, 20 and 30 issuch that the pulsation of cogging torque of the rotor block 10 advancesthe phase at an electrical angle equivalent to mechanical angle θ1 withrespect to the rotor block 20, while the pulsation of cogging torque ofthe rotor block 30 delays the phase at an electrical angle equivalent tomechanical angle θ2 with respect to the rotor block 20.

If the pulsation of cogging torque of each of the rotor blocks 10, 20and 30 is a sinusoidal wave and the mutual displacement angle of therotor blocks 10, 20 and 30 is corresponding to an electrical angle of120° of a pulsation period of cogging torque (one third of the pulsationperiod) generated in the rotor which is not divided, then the pulsationsof cogging torques of the rotor blocks 10, 20 and 30 are combined in thesame manner as three-phase sinusoidal alternating currents, andcancelled out.

FIG. 12A is a waveform chart of cogging torque of each of the rotorblocks 10, 20 and 30 in this case and shows the waveforms of coggingtorque Ta in the rotor block 10, cogging torque Tb in the rotor block 20and cogging torque Tc in the rotor block 30. In FIG. 12A, θ1 indicates aleading phase angle obtained by advancing the waveform of the coggingtorque Tb at an electrical angle equivalent to the mechanical angle θ1shown in FIG. 11, while θ2 indicates a lagging phase angle obtained bydelaying the waveform of the cogging torque Tb at an electrical angleequivalent to the mechanical angle θ1 shown in FIG. 11 (they aredesignated with the same codes in FIG. 12A for the purpose offacilitating the comparison with FIG. 11). In addition, θ11 representsthe pulsation period of the cogging torque waveform. FIG. 12B indicatesthe composite cogging torque Tcs thereof, and, as explained in above,pulsation does not appear in the composite cogging torque Tcs resultingfrom combining the three-phase sinusoidal waves of the cogging torquesTa, Tb and Tc.

FIG. 13A is a waveform chart of each cogging torque when distortion ispresent in the cogging torques of the rotor blocks 10, 20 and 30. FIG.13B indicates the composite cogging torque Tcs thereof. In the casewhere the cogging torques of the rotor blocks 10, 20 and 30 include evenhigher harmonics as explained in the conventional example, if thecomposite cogging torque Tcs is shown by an expression similar toexpression (1), it is given by the following expression (2), where x isan electrical angle representing the angle of an arbitrary rotationalposition of the rotor and n is a natural number. According to thisexpression (2), not only the fundamental wave components of coggingtorques, but also the even harmonics components thereof cancel eachother out, so that pulsation of cogging torque will not appeartheoretically as shown in FIG. 13B. $\begin{matrix}\begin{matrix}{{Tcs} = {{{TO}/3}\left\{ {{\sin(x)} + {\sum\limits_{n = 1}^{\infty}\quad\left\lbrack {{kn} \cdot {\sin\left( {2{nx}} \right)}} \right\rbrack} + {\sin\left( {x + {2{\pi/3}}} \right)} +} \right.}} \\{{\sum\limits_{n = 1}^{\infty}\quad\left\lbrack {{kn} \cdot {\sin\left( {{2{nx}} + {2{n \cdot 2}{\pi/3}}} \right)}} \right\rbrack} +} \\\left. {{\sin\left( {x + {4{\pi/3}}} \right)} + {\sum\limits_{n = 1}^{\infty}\quad\left\lbrack {{kn} \cdot {\sin\left( {{2{nx}} + {2{n \cdot 4}{\pi/3}}} \right)}} \right\rbrack}} \right\} \\{= 0}\end{matrix} & (2)\end{matrix}$(Second Embodiment)

FIGS. 14A, 14B and 14C show the construction of a rotor of a brushlessDC motor according to the second embodiment of the present invention bya plan view (FIG. 14A) and cross sectional views (FIGS. 14B and 14C) ina direction perpendicular to the rotary shaft. This rotor is a permanentmagnet-buried type rotor constructed by inserting permanent magnets intoempty holes formed in a rotor core, and comprises a rotor core 17 andfour sets of permanent magnets fitted at equal intervals into four emptyholes 19 formed in the outer circumferential portion of the rotor core17 at equal intervals along the outer circumference.

Each of the four sets of permanent magnets is composed of threepermanent magnets 16, 26 and 36 located in the upper, middle and lowerstages, the upper stage being one end in the direction of the rotaryshaft. The permanent magnets 16, 26 and 36 have the same size. The rotorcore 17 is formed by layering a number of thin electromagnetic steelplates and fixing them integrally, and provided with four couplingmember insertion holes 18 for fixing them integrally with couplingmembers.

In FIGS. 14A, 14B and 14C, a common central straight line K1 is drawnthrough the permanent magnets 16, 26 and 36 to show the displacementangle of the respective magnets. The upper-stage permanent magnet 16 isdisplaced by counterclockwise rotation only at an angle θ1 formed by acentral line J1 of the permanent magnet 16 and the common centralstraight line K1. The lower-stage permanent magnet 36 is displaced byclockwise rotation only at an angle θ2 formed by a central line J2 ofthe permanent magnet 36 and the common central straight line K1. Themiddle-stage permanent magnet 26 is not displaced, and the central lineof the permanent magnet 26 coincides with the common central straightline K1. The mutual displacement angles θ1, θ2 of the permanent magnets16, 26 and 36 are corresponding to an electrical angle of b 120° of apulsation period of cogging torque (one third of the pulsation period)generated when the permanent magnets 16, 26 and 36 are not displaced.

Here, the effective polar opening angle of the permanent magnets 16, 26and 36 is denoted as θm and the pole pitch angle of the permanentmagnets 16, 26 and 36 is denoted as θp. In order to effectively utilizethe present invention, if the respective values are set so as to satisfythe relationship of the following expression (3), then at least theeffective polar opening angle θm of the rotor obtained by layering thepermanent magnets 16, 26 and 36 at predetermined displacement anglesaccording to the point of the present invention will not exceed the polepitch angle θp of the permanent magnets 16, 26 and 36. Therefore, everymagnetic flux of the permanent magnets 16, 26 and 36 used in the rotoris utilized as the effective magnetic flux.θp≧θm+θ1+θ2+. . .   (3)

Moreover, as explained in the first embodiment, if the effectivemagnetic polar opening angle θm of the rotor obtained by constructingthe rotor blocks according to the point of the present invention is setso as not to exceed at least the pole pitch angle θp of the permanentmagnets, then the pole pitch angle θp of the rotor includes theeffective polar opening angle θm of the permanent magnets positioned inthe individual rotor blocks and θ1+θ2 that is the sum of thedisplacement angles of the rotor blocks. Thus, there is no need todisplace the rotor cores in the circumferential direction. Accordingly,by only displacing the positions of the permanent magnets divided intothree blocks in the rotation axis direction, in the circumferentialdirection at a mechanical angle equivalent to one third of a pulsationperiod caused by cogging of the rotor which is not divided, it ispossible to obtain the same effects as the construction where the threedivided rotor blocks are layered while mutually displacing them.

In a type of a rotor shown in FIG. 11 in which permanent magnets arearranged on the outer circumferential surface of a rotor yoke (rotorcore), a central angle corresponding to the outer circumferentialportion of the effective magnetic pole and a central angle correspondingto the outer circumferential portion of the permanent magnet are equalto each other. On the other hand, a permanent magnet-buried type rotoras shown in FIGS. 14A, 14B and 14C does not especially aim to have astructure for converging the magnetic flux. However, if the proportionof the magnetic material portion of the rotor core located between thepermanent magnets and the outer circumference of the rotor is large, themagnetic flux of the permanent magnet tends to diffuse in this magneticmaterial portion. Therefore, the central angle corresponding to theouter circumferential portion of the effective magnetic pole tends to bewider than the central angle corresponding to the outer circumferentialportion of the permanent magnet. The central angle corresponding to theouter circumferential portion of the permanent magnet can be madesmaller by an angle equivalent to this diffusion. As a result, it ispossible to set a larger space between the permanent magnets in thecircumferential direction.

Conventionally, for this type of the rotor, skew-shaped permanentmagnets or ring-shaped permanent magnets magnetized on the skew are usedas cogging torque preventing means. The magnetization of such apermanent magnet is carried out by skew magnetization using a specialmagnetic yoke before incorporating the rotor into a case or the like. Ina brushless DC motor according to the present invention, the centralangle em corresponding to the outer circumferential portion of theeffective magnetic pole of each rotor block and the displacement angleof each rotor block are set so as not to exceed the central angle θpcorresponding to the circumferential interval between the magnetic polesof the rotor. Therefore, after assembling the motor, the rotor blocks orthe rotor can be magnetized by causing a direct current to flow in thestator and using the stator as the magnetic yoke. It is thus possible toachieve built-in magnetization of magnetizing the rotor in anon-magnetic state.

Note that the above-described first and second embodiments areillustrated on condition that the rotor or the permanent magnets areequally divided, but, if the changes in cogging torques of the dividedrotor blocks or the rotor portions of the divided permanent magnets aresubstantially the same, it is not necessarily to divide the rotor andthe permanent magnets equally, and it is needless to say that thematerials of the permanent magnets can be different from each other.Further, while the first and second embodiments are illustrated withreference to a rotor constructed by burying permanent magnets as shownin FIGS. 14A, 14B, 14C near its surface along the outer circumference ofthe rotor core, it is needless to explain that the present invention isalso applicable to rotors constructed by burying permanent magnets inconcave-, V-, reversed circular arc- or flat plate-shape in the rotorcore.

(Third Embodiment)

As shown in FIG. 15, the third embodiment is implemented by applying thepresent invention to a brushless DC motor comprising a magnet-buriedtype rotor 5 having four magnetic poles. In other words, the rotor 5 ofthis brushless DC motor has magnet mounting holes 8 formed at fourpositions along the outer circumference, and permanent magnets 7 aremounted in the magnet mounting holes 8, respectively. The permanentmagnets 7 are arranged adjacent to the outer circumference of the rotor5. For this reason, it can be said that the opening angle of eachpermanent magnet 7 seen from a rotational center axis θ is equal to theeffective polar opening angle. The permanent magnets 7 are arranged sothat adjacent permanent magnets 7 have mutually opposite polarities. Astator 1 comprises a yoke 2 as an outer circumferential portion, andtwelve teeth 3 that are provided at equal intervals and protrude fromthe yoke 2 toward the center. Adjacent teeth 3 form a slot 4 togetherwith the yoke 2. Note that only the upper half portion is shown in FIG.15 for simplification, but the lower half portion also has the sameconstruction. Further, although not shown in FIG. 15, the teeth 3 areprovided with windings in actual use.

In FIG. 15, J3 and J4 indicate the central lines of the magnet mountingholes 8, i.e., the pole central lines. They are owing to the corestructure of the rotor 5. K3 and K4 indicate the central lines of thepermanent magnets 7 a and 7 b, respectively, i.e., the central lines ofthe effective polar opening angles of the respective magnetic poles.They represent the center position of actual magnetic force. θ3indicates the pitch angle owing to the structure of the rotor 5 seenfrom the rotational center axis O. θ4 indicates the effective polaropening angle of the permanent magnet 7 a. θ5 indicates the slot pitchangle of the stator 1. θ6 indicates the deviation angle between the polecentral line J3 of the magnetic pole of the permanent magnet 7 a and thecentral line K3 of the effective polar opening angle. θ7 indicates theslot opening angle that is the opening angle between adjacent teeth 3.Here, one of the lines defining both ends of the slot pitch angle θ5coincides with the central line K3 of the effective polar opening angle,but this is merely an accident and has no special meaning. Similarly, itis merely an accident that the other of the lines defining both ends ofthe slot pitch angle θ5 coincides with one of the lines defining bothends of the effective polar opening angle θ4. Furthermore, it is merelyan accident that one of the lines defining both ends of the slot openingangle θ7 coincides with one of the lines defining both ends of theeffective polar opening angle θ4.

According to the brushless DC motor shown in FIG. 15, in the magneticpole of the permanent magnet 7 a, there is an angular deviation betweenthe pole central line J3 and the central line K3 of the effective polaropening angle, and the value of the deviation angle is θ6. On the otherhand, in the magnetic pole of the permanent magnet 7 b, the pole centralline J4 and the central line K4 of the effective polar opening anglecoincide with each other. In other words, the value of the deviationangle between them is zero. Thus, the two magnetic poles have adifference of θ6 in the deviation angle between the pole central lineand the central line of the effective polar opening angle.

Referring to FIGS. 16 and 17, the following description will explain thestate of cogging torque of the brushless DC motor having theabove-mentioned construction. FIG. 16 shows the teeth 3 (3-1, 3-2, . . ., 3-n, 3-(n+1)), the magnet mounting holes 8 and the permanent magnets 7b and 7 a of the brushless DC motor of FIG. 15 by linearly developingthem. FIG. 17 shows the waveforms of cogging torques generated by thebrushless DC motor. The upper row is a graph when the deviation angle θ6is equal to the slot opening angle θ7 in the brushless DC motor of FIG.15. The lower row is shown for a comparison purpose, and is a graph whenthe deviation angle θ6 is zero. Note that since θ4=θ5, in order toestablish the relation 0.2×θ7≦θ6≦θ5−(0.2×θ7), it is necessary to satisfyθ7≦(5/6)θ5. However, since θ7 is usually equal to about a half of θ5 orless, the above expression would be established.

Let consider how the permanent magnets 7 b and 7 a move from the stateshown in FIG. 16 toward the right direction in FIG. 16 with the rotationof the rotor. At this time, the left end of the permanent magnet 7 breceives an attraction force from the tooth 3-1. This attraction forceacts as interference to the movement of the permanent magnet 7 b in theright direction. Then, when the left end of the permanent magnet 7 breaches a position closer to the tooth 3-2 than to the tooth 3-1, theleft end of the permanent magnet 7 b receives a force in a direction ofassisting the movement. The reason for this is that the attraction forcefrom the tooth 3-2 becomes dominant. Thus, the left end of the permanentmagnet 7 b generates cogging torque that repeats interference andassistance periodically with respect to the movement.

Similarly, the left end of the permanent magnet 7 a generates coggingtorque. However, there is a time difference between a timing in whichthe left end of the permanent magnet 7 a receives a force in aninterfering direction because of the relationship with the tooth 3-n anda timing in which the left end of the permanent magnet 7 b receives aforce in the interfering direction because of the relationship with thetooth 3-1. The time difference is a lag corresponding to the deviationangle θ6. There is also a time difference between the timings ofreceiving the force in the assisting direction. Hence, there is a phaseangle equivalent to the deviation angle θ6 between the cogging torquegenerated by the left end of the permanent magnet 7 b and the coggingtorque generated by the left end of the permanent magnet 7 a.

The graph in the upper row of FIG. 17 shows this state when thedeviation angle θ6 is equal to the slot opening angle θ7. In otherwords, the thin solid line of this graph indicates cogging torque TC1generated by the left end of the permanent magnet 7 b. The broken lineindicates cogging torque TC2 generated by the left end of the permanentmagnet 7 a. Further, the thick solid line indicates their compositecogging torque TC0. The amplitude of the cogging torque TC0 is muchsmaller than the sum of the amplitudes of the cogging torques TC1 andTC2. The reason for this is that there is a time difference between thepeaks of the cogging torques TC1 and TC2 due to the phase differencetherebetween and most of the torques cancel each other out.

If this state is compared with the graph in the lower row of FIG. 17(obtained when the deviation angle θ6 is zero), the effect can beclearly understood. In other words, in the graph of the lower row, theamplitude of the cogging torque TC0 is equal to the sum of theamplitudes of the cogging torques TC1 and TC2 (which are equal to eachother in the lower row of FIG. 17). There is no phase difference betweenthe cogging torques TC1 and TC2, and thus the cogging torques TC1 andTC2 have the peaks simultaneously and the torques are added together asit is. In other words, the effect of the third embodiment is that theamplitude of the cogging torque TC0 in the graph of the upper row ismuch smaller than the amplitude of the cogging torque TC0 in the graphof the lower row.

Note that this type of brushless DC motor is generally designed so as tomake the slot opening angle θ7 as small as possible. The reason for thisis to make the magnetic resistance between the permanent magnets and theteeth as small as possible and ensure a large interlinkage magneticflux. On the other hand, this makes the change rate (the maximuminclination of the curves shown in the graph of FIG. 17) of theindividual cogging torques (corresponding to TC1 and TC2) generated bythe respective permanent magnets steep. Here, if the deviation angle θ6between the permanent magnets 7 b and 7 a is equal to the slot openingangle θ7, the abrupt changes of the individual cogging torques areeffectively cancelled out. Therefore, the composite cogging torque(corresponding to TC0) becomes smaller. This is the reason why the upperrow of FIG. 17 shows the example when θ6=θ7. Further, if the slotopening angle θ7 is equal to one second of the slot pitch angle θ5, thecogging torques TC1 and TC2 establish an antiphase relationship. In thiscase, the two cogging torques cancel each other out extremely well.

FIG. 18 is a graph showing the relationship between the deviation angleθ6 and the peak value of the composite cogging torque. In this graph,the horizontal axis indicates the deviation angle θ6, and the deviationangle θ6 is within a range from zero to the slot pitch angle θ5. If thedeviation angle θ6 is equal to the slot pitch angle θ5, since thepermanent magnet deviates by an amount corresponding to one slot, thisstate is the same as a state where there is no deviation. The verticalaxis indicates the pitch value of the composite cogging torque. Thescale on the vertical axis shows a reference value by denoting a valueobtained when the deviation angle θ6 is zero as 1. Moreover, in FIG. 18,two curves TC0-1 (solid line) and TC0-2 (alternate long and short dashline) are drawn. These two curves resulted from the difference betweenthe waveforms of the individual cogging torques (corresponding to TC1and TC2). The curve TC0-1 represents the state where there is adifference between the waveform at the time of rising and the waveformat the time of falling, i.e., even harmonics components are included.The curve shown in FIG. 17 belongs to this state though it is no soapparent. In an actual fact, this state occurs more frequently. Thecurve TC0-2 represents the state where the waveform at the time ofrising and the waveform at the time of falling are symmetrical, i.e.,even higher harmonics components are not included.

The following will be understood from the graph of FIG. 18. Regardingthe curve TC0-1, at both ends (θ6=0, θ6=θ5), the value on the verticalaxis is 1. The reason for this is that there is no deviation. At aslightly inward position in the direction of the horizontal axis, thevalue on the vertical axis abruptly decreases. This occurs because ofthe cancellation effect of cogging torques caused by the deviation.After coming to a position where the distances from both ends are 0.2times the slot opening angle θ7 (θ6=0.2×θ7, θ6=θ5−(0.2×θ7)), the changeof the value on the vertical axis becomes moderate at further inwardpositions. The reason for this is that the cogging torques do notcompletely cancel each other out because of the difference between thewaveform at the time of rising and the waveform at the time of falling.Within this range, the value on the vertical axis has a minimum at thecenter in the direction of the horizontal axis (θ6=θ0.5×θ5), that is,substantially a half of the values at both ends. Thus, it can beunderstood that the range of the deviation angle θ6 for effectivelyobtaining the cogging torque reducing effect is 0.2×θ7≦θ6≦θ5−(0.2×θ7).

Moreover, in view of the curve TC0-2 in the graph of FIG. 18, there isno big difference from the curve TC0-1 in the vicinity of both ends.However, even within the range of 0.2×θ7≦θ6≦θ5−(0.2×θ7), the value onthe vertical axis further decreases considerably. At the center(θ6=0.5×θ5), the value on the vertical axis becomes almost zero. Thereason for this is that the cogging torques are more certainly cancelledout because the waveform at the time of rising and the waveform at thetime of falling are symmetrical.

In the above-described manner, the overall cogging torque of thebrushless DC motor of FIG. 15 is reduced by the deviation angle θ6.Moreover, in the brushless DC motor of FIG. 15, a magnetic pole(permanent magnet 7 b) with a deviation angle of zero and a magneticpole (permanent magnet 7 a) with a deviation angle of θ6 are arrangednext to each other. Therefore, even though the permanent magnet 7 a hasthe deviation angle θ6, the center of gravity of the rotor 5 does notmuch deviate from the center of the axis. Therefore, the rotary balanceof the rotary object is not affected very much. Thus, the rotation isperformed smoothly.

Note that while FIG. 15 shows the upper half portion of the rotor 5, itis more preferable to arrange one magnetic pole with a deviation angleof zero and one magnetic pole with the deviation angle θ6 in its lowerhalf portion in the same manner as in the upper half portion as shown inFIG. 19. Accordingly, two magnetic poles with a deviation angle of zeroand two magnetic poles with the deviation angle θ6 are present in therotor 5 as a whole, i.e., the number of the respective magnetic polesare equal to each other. Therefore, the reduction in cogging torque ofthe brushless DC motor as a whole is achieved more satisfactorily.Further, in the brushless DC motor shown in FIG. 19, magnetic poles witha deviation angle of zero and magnetic poles with the deviation angle θ6are arranged at symmetrical positions. Accordingly, in thecircumferential direction, the magnetic pole with a deviation angle ofzero and the magnetic pole with the deviation angle θ6 are presentalternately. Thus, the influence of the deviation angle on the positionof the center of gravity of the rotor 5 is cancelled. Consequently, thecenter of gravity coincides with the center of the axis, and the rotarybalance is very well.

(Fourth Embodiment)

The fourth embodiment adopts a block construction in which the rotor isdivided in the axial direction. Here, an example in which the rotor isdivided into two blocks will be explained. The rotor of the fourthembodiment comprises a block shown in FIG. 20A and a block shown in FIG.20B. The number of magnetic poles in each block is b 4, that is the sameas in FIG. 15. Note that since the stator is the same as that shown inFIG. 15, it is omitted in FIGS. 20A and 20B.

In the block of FIG. 20A, the deviation angle in each magnetic pole isas follows. In the magnetic pole (permanent magnet 711) shown at theupper left in FIG. 20A, there is a deviation of angle θ6 between a polecentral line J3-1 and an effective polar opening angle central lineK3-1. In the magnetic pole (permanent magnet 712) shown at the upperright in FIG. 20A, a pole central line J4-1 and an effective polaropening angle central line K4-1 coincide with each other. In themagnetic pole (permanent magnet 713) shown at the lower right in FIG.20A, like the upper left magnetic pole, there is a deviation of angle θ6between a pole central line J5-1 and an effective polar opening anglecentral line K5-1. In the magnetic pole (permanent magnet 714) shown atthe lower left in FIG. 20A, like the upper right magnetic pole, a polecentral line J6-1 and an effective polar opening angle central line K6-1coincide with each other. In short, two magnetic poles with a deviationangle of zero and two magnetic poles with the deviation angle θ6 arepresent, and they are arranged alternately in the circumferentialdirection.

In the block of FIG. 20B, the deviation angle in each magnetic pole isas follows. In the magnetic pole (permanent magnet 721) shown at theupper left in FIG. 20B, a pole central line J3-2 and an effective polaropening angle central line K3-2 coincide with each other. In themagnetic pole (permanent magnet 722) shown at the upper right in FIG.20B, there is a deviation of angle θ6 between a pole central line J4-2and an effective polar opening angle central line K4-2. In the magneticpole (permanent magnet 723) shown at the lower right in FIG. 20B, likethe upper left magnetic pole, a pole central line J5-2 and an effectivepolar opening angle central line K5-2 coincide with each other. In themagnetic pole (permanent magnet 724) shown at the lower left in FIG.20B, there is a deviation of angle θ6 between a pole central line J6-2and an effective polar opening angle central line K6-2 like the upperright magnetic pole. In short, two magnetic poles with a deviation angleof zero and two magnetic poles with the deviation angle θ6 are present,and they are arranged alternately in the circumferential direction. Thisis the same as the previously illustrated structure in FIG. 19.

It can be seen by comparing these two blocks that the presence andabsence of a deviation angle are the opposite between the correspondingmagnetic poles in the axial direction. Therefore, the brushless DC motorof the fourth embodiment produces the following effects. Precisely, ineach of the two blocks, as explained with reference to FIG. 19 above,the cogging torque is satisfactorily reduced and an excellent rotarybalance is obtained. In addition, since the presence and absence of adeviation angle differ between the corresponding magnetic poles in theaxial direction, the following effects are obtained. Namely, in therotor as a whole, within a single magnetic pole (for example, thepermanent magnet 711 and the permanent magnet 721, etc.), the coggingtorques cancel each other out. Therefore, in the brushless DC motor as awhole, the individual cogging torques are respectively reduced.Consequently, the overall cogging torque is extremely small.

Moreover, regarding the rotary balance, the influence of he deviation islocally reduced at respective positions of the rotor. Therefore, thisbrushless DC motor is suitable for applications for abrupt accelerationor abrupt deceleration and also an application used in high-speedrotation. Further, in the corresponding magnetic poles in the axialdirection, the magnetic poles of the permanent magnets are equal to eachother. They include magnetic poles having a deviation and magnetic polehavings no deviation. For this reason, in the brushless DC motor as awhole, the effective polar opening angle is substantially widened.Moreover, the pole pitch of the rotor itself is an equal interval.Furthermore, in the gap between the rotor and the stator, the change inthe magnetic flux density in the circumferential direction is smooth.

(Fifth Embodiment)

In the fifth embodiment, the number of magnetic poles in thecircumferential direction of the rotor is made six. The rotor of thefifth embodiment has the construction shown in FIG. 21. In other words,a rotor 51 has magnet mounting holes 81 to 86 at six positions along theouter circumference, and permanent magnets 71 to 76 are mounted in themagnet mounting holes 81 to 86, respectively. The permanent magnets 71to 76 are arranged so that the magnetic poles of adjacent permanentmagnets are opposite to each other. Note that since there is noparticular difference between the stator of this embodiment and the oneshown in FIG. 15, the stator is omitted in FIG. 21.

In the rotor 51, the deviation angle in each magnetic pole is asfollows. In the magnetic pole (permanent magnet 71) shown on the leftside in FIG. 21, a pole central line J11 and an effective polar openingangle central line K11 coincide with each other. In the magnetic pole(permanent magnet 72) shown at the upper left in FIG. 21, there is adeviation of angle θ6 between a pole central line J12 and an effectivepolar opening angle central line K12. If the rotary direction of therotor 51 is clockwise, then the line K12 is located on a position aheadthe line J12 by an amount corresponding to the angle θ6. In the magneticpole (permanent magnet 73) shown at the upper right in FIG. 21, there isa deviation of angle θ6 between a pole central line J13 and an effectivepolar opening angle central line K13. However, the direction of thedeviation is opposite to that in the upper left magnetic pole, and thusthe line K13 is located on a position behind the line J13 by an amountcorresponding to the angle θ6. In the magnetic pole (permanent magnet74) shown on the right side in FIG. 21, a pole central line J14 and aneffective polar opening angle central line K14 coincide with each otherlike the left side magnet pole. In the magnetic pole (permanent magnet75) shown at the lower right in FIG. 21, there is a deviation of angleθ6 between a pole central line J15 and an effective polar opening anglecentral line K15 in the leading direction like the upper left magneticpole. In the magnetic pole (permanent magnet 76) shown at the lower leftin FIG. 21, there is a deviation of angle θ6 between a pole central lineJ16 and an effective polar opening angle central line K16 in the laggingdirection like the upper right magnetic pole.

In short, two magnetic poles with a deviation angle of zero, twomagnetic poles with the deviation angle θ6 in the leading direction, andtwo magnetic poles with the deviation angle θ6 in the lagging directionare present, and thus the number of the respective magnetic poles arethe same. Magnetic poles having a deviation angle other than these threedeviation angles do no exist. Further, magnetic poles having a deviationangle in mutually opposite directions are arranged on both sides (theleft and right sides) of a magnetic pole having a deviation angle ofzero.

In the brushless DC motor comprising the rotor 51 of the above-describedstructure, the cancellation of cogging torques is performed as shown inthe waveform chart of FIG. 22. In the waveform chart of FIG. 22, thethin solid line indicates cogging torque TC11 generated by the left andright magnetic poles in FIG. 21. The broken line indicates coggingtorque TC12 generated by the upper right and lower left magnetic polesin FIG.21. The alternate long and short dash line indicates coggingtorque TC13 generated by the upper left and lower right magnetic polesin FIG. 21. Moreover, the thick solid line indicates composite coggingtorque TC10 resulting from combining them. Note that FIG. 21 shows thewaveforms when the magnitude of the deviation angle θ6 is equal to onethird of the slot pitch angle θ5. Thus, in the fifth embodiment, thecancellation of cogging torques is carried out by three-phasecomposition. Therefore, the composite cogging torque TC10 shown in FIG.21 is almost zero. Even when the individual cogging torques areasymmetrical waveforms containing even higher harmonics components, theoverall cogging torque is very satisfactorily reduced. Moreover, evenwhen there is slight distortion in the waveforms or there is a slightvariation in the magnitude of the individual cogging torques, the effectis stable.

Here, if the rotor 51 of FIG. 21 is seen carefully, it can be understoodthat three adjacent magnetic poles in the circumferential direction cannever include magnetic poles having the same deviation angle (includingthe direction). This fact is established for any three adjacent magneticpoles. In the rotor 51, therefore, the influence of the deviation on therotary balance is locally eliminated in the respective positions.Consequently, the brushless DC motor of the fifth embodiment hasexcellent rotary performance.

(Sixth Embodiment)

The sixth embodiment shown in FIGS. 23A and 23B is a combination of thefourth and fifth embodiments. In other words, the sixth embodimentcomprises six magnetic poles and employs a divided-block structure. Arotor of the sixth embodiment comprises a block shown in FIG. 23A and ablock shown in FIG. 23B. Note that the illustration of the stator isalso omitted in the sixth embodiment.

In the block of FIG. 23A, the deviation angle in each magnetic pole isas follows. In the magnetic pole (permanent magnet 71-1) shown on theleft side in FIG. 23A, a pole central line J11-1 and an effective polaropening angle central line K11-1 coincide with each other. In themagnetic pole (permanent magnet 72-1) shown at the upper left in FIG.23A, there is a deviation of angle θ6 in the leading direction between apole central line J12-1 and an effective polar opening angle centralline K12-1. In the magnetic pole (permanent magnet 73-1) shown at theupper right in FIG. 23A, there is a deviation of angle θ6 in the laggingdirection between a pole central line J13-1 and an effective polaropening angle central line K13-1. In the magnetic pole (permanent magnet74-1) shown on the right side in FIG. 23A, a pole central line J14-1 andan effective polar opening angle central line K14-1 coincide with eachother. In the magnetic pole (permanent magnet 75-1) shown at the lowerright in FIG. 23A, like the upper left magnetic pole, there is adeviation of angle θ6 in the leading direction between a pole centralline J15-1 and an effective polar opening angle central line K15-1. Inthe magnetic pole (permanent magnet 76-1) shown at the lower left inFIG. 23A, like the upper right magnetic pole, there is a deviation ofangle θ6 in the lagging direction between a pole central line J16-1 andan effective polar opening angle central line K16-1. This is the same asthat shown in FIG. 21.

In the block of FIG. 23B, the deviation angle in each magnetic pole isas follows. In the magnetic pole (permanent magnet 71-2) shown on theleft side in FIG. 23B, there is a deviation of angle θ6 in the laggingdirection between a pole central line J11-2 and an effective polaropening angle central line K11-2. In the magnetic pole (permanent magnet72-2) shown at the upper left in FIG. 23B, a pole central line J12-2 andan effective polar opening angle central line K12-2 coincide with eachother. In the magnetic pole (permanent magnet 73-2) shown at the upperright in FIG. 23B, there is a deviation of angle θ6 in the leadingdirection between a pole central line J13-2 and an effective polaropening angle central line K13-2. In the magnetic pole (permanent magnet74-2) shown on the right side in FIG. 23B, like the left side magneticpole, there is a deviation of angle θ6 in the lagging direction betweena pole central line J14-2 and an effective polar opening angle centralline K14-2. In the magnetic pole (permanent magnet 75-2) shown at thelower right in FIG. 23B, like the upper left magnetic pole, a polecentral line J15-2 and an effective polar opening angle central lineK15-2 coincide with each other. In the magnetic pole (permanent magnet76-2) shown at the lower left in FIG. 23B, like the upper right magneticpole, there is a deviation of angle θ6 in the leading direction betweena pole central line J16-2 and an effective polar opening angle centralline K16-2.

It can be seen by comparing these two blocks that the correspondingmagnetic poles in the axial direction absolutely have differentdeviation angles. Therefore, the brushless DC motor of the sixthembodiment produces the following effects. Namely, in these two blocks,as explained with reference to FIG. 21 (the fifth embodiment) above, thecogging torque is satisfactorily reduced and an excellent rotary balanceis achieved. In addition, the effects explained in FIGS. 20A and 20B(the fourth embodiment) are also obtained because of the difference inthe deviation angle between the corresponding magnetic poles in theaxial direction. In other words, in the rotor as a whole, thecancellation of cogging torques and rotary unbalance is performed evenwithin a single magnetic pole. Consequently, the overall cogging torqueis extremely small, and the rotary balance is excellent.

(Seventh Embodiment)

In the seventh embodiment, instead of providing a deviation in thepositions of permanent magnets, cancellation of cogging torques betweenthe magnetic poles is achieved by other means. The other means isimplemented by providing convex portions corresponding to the magneticpoles on the periphery of the rotor core and shifting the position ofthe convex portion in each magnetic pole. A rotor 52 of the seventhembodiment has the construction shown in FIG. 24. The rotor 52 has fourmagnetic poles. However, unlike the third through sixth embodiments,each permanent magnet is mounted in the center of each magnet mountinghole. In other words, in every magnetic pole, the pole central line(J21, etc.) and the effective polar opening angle central line (K21,etc.) coincide with each other.

However, the rotor 52 has convex portions 61 to 64 corresponding to therespective magnetic poles on its outer circumference. In FIG. 24, L1 toL4 represent the central lines of the respective convex portions 61 to64 seen from the rotational center axis O. Moreover, in the convexportion 61 shown at the upper left in FIG. 24, the convex portioncentral line Li coincides with a pole central line J21 and an effectivepolar opening angle central line K21. In the convex portion 62 shown atthe upper right in FIG. 24, the convex portion central line L2 islocated on a position deviated from a pole central line J22 and aneffective polar opening angle central line K22, and the deviation angleis θ6. In the convex portion 63 shown at the lower right in FIG. 24,like the convex portion 61, the convex portion central line L3 coincideswith a pole central line J23 and an effective polar opening anglecentral line K23. In the convex portion 64 shown at the lower left inFIG. 24, like the convex portion 62, the convex portion central line L4is located on a position deviated at the angle θ6 from a pole centralline J24 and an effective polar opening angle central line K24. Theconvex portions 61 to 64 in FIG. 24 are drawn in an exaggerated mannerto facilitate understanding, and, in an actual fact, the difference inlevel caused by the convex portions is very small.

In a brushless DC motor using the rotor 52, in each magnetic pole, thegap between the rotor and the stator is smaller and the magneticresistance is smaller in a position where the convex portion is presentthan in the outside of this position. Therefore, magnetic flux convergeson this position and the positions including the convex portionsdominantly contribute to the generation of cogging torque. Thus,similarly to explanation given in the third embodiment, etc., the effectof reducing the cogging torque is produced by the presence of the convexportions having a deviation angle and the convex portions having nodeviation angle.

Moreover, in the seventh embodiment, since no deviation is introduced inthe permanent magnets, the following effects are also produced.Precisely, the influence of the deviation of the convex portions on theposition of the center of gravity of the rotor is much smaller than thatof the permanent magnets. Therefore, an extremely good rotary balance isobtained. Furthermore, the absence of deviation in the permanent magnetsallows the use of the largest possible permanent magnet within the rangeof the magnet mounting hole. In addition, it is also possible to allowthe permanent magnet to fully occupy the space within the pole pitch. Inthis case, stronger rotary force is obtained. Even if such a largepermanent magnet is not used, the seventh embodiment has a merit thatthe degree of freedom in designing the brushless DC motor is high.Besides, the cogging torque generated by a single magnetic pole is alsosmaller compared to one without convex portion. The reason for this isthat an end of a permanent magnet and an end of a convex portiongenerate cogging torques, respectively, and there is a phase differencebetween them.

(Eighth Embodiment)

In the eighth embodiment, deviations are introduced in both of thepermanent magnets and the convex portions. A rotor 53 of the eighthembodiment is constructed as shown in FIG. 25. In the rotor 53, thedeviation angle in each magnetic pole is as follows. In the magneticpole (convex portion 611) shown at the upper left in FIG. 25, all of apole central line J31, an effective polar opening angle central line K31and a convex portion central line L11 coincide with each other. In themagnetic pole (convex portion 612) shown at the upper right in FIG. 25,both of an effective polar opening angle central line K32 and a convexportion central line L12 are located on a position deviated from a polecentral line J32 at the angle θ6. In the magnetic pole (convex portion613) shown at the lower right in FIG. 25, like the upper left magneticpole, all of a pole central line J33, an effective polar opening anglecentral line K33 and a convex portion central line L13 coincide witheach other. In the magnetic pole (convex portion 614) shown at the lowerleft in FIG. 25, like the upper right magnetic pole, both of aneffective polar opening angle central line K34 and a convex portioncentral line L14 are located on a position deviated from a pole centralline J34 at the angle θ6. In the (upper right and lower left) magneticpoles having a deviation, the deviation angle of the convex portion andthe deviation angle of the permanent magnet are both θ6. Therefore, inany magnetic pole, the convex portion is positioned at the center of thepermanent magnet.

In the rotor 53, a reduction in cogging torque by the magnetic poleshaving a deviation and the magnetic poles having no deviation isachieved by both of the permanent magnets and the convex portions.Moreover, the coincidence of the center of the permanent magnet and thecenter of the convex portion in each magnetic pole produces thefollowing effects. First, the magnetic force of the permanent magnets ismore effectively utilized. This effect is produced by the presence ofthe convex portion at the center of the permanent magnet. Here, it isalso possible to cancel the cogging torques, depending on therelationship between the slot pitch angle and the angular differencebetween an end of the permanent magnet and an end of the convex portion.

(Modified Examples of Third through Eighth Embodiments)

Next, the following description will explain modified examples of theconfigurations of the rotor core and the permanent magnet. A modifiedexample shown in FIG. 26 is an example of application to a structurehaving a relatively long distance between the outer circumference of therotor core and the permanent magnets. In the rotor of FIG. 26, linearpermanent magnets are used. Further, there are substantially bow-shapemagnetic regions 65 to 68 between the respective permanent magnets andthe outer circumference of the rotor. The periphery of the magneticregions 65 to 68 form effective polar opening angles M1 to M4. In thisrotor, the permanent magnets are arranged at an equal pitch, and nodeviation is introduced. However, there is a difference in theconfiguration among the magnetic regions 65 to 68. Specifically, in theupper right and lower left magnetic regions 66 and 68, the right sideand the left side are symmetrical about a pole central line J.Therefore, the pole central line J and an effective polar opening anglecentral line K coincide with each other. However, in the upper left andlower right magnetic regions 65 and 67, the right side and the left sideare asymmetrical. Hence, there is a deviation angle θ6 between the polecentral line J and the effective polar opening angle central line K.Thus, a reduction in cogging torque is achieved by the presence orabsence of a deviation angle in each magnetic pole.

FIG. 27 shows an example implemented by replacing the permanent magnetsof FIG. 26 with curved permanent magnets. In this rotor, there is alsothe presence or absence of a deviation angle in each magnetic polebecause of the difference in the configuration among the magneticregions between the permanent magnets and the outer circumference of therotor. Thus, a reduction in cogging torque is achieved. The modifiedexamples shown in FIG. 26 or FIG. 27 are applicable to any one of thethird through eighth embodiments. It is not necessarily to limit thepermanent magnets to those shown in FIG. 26 or FIG. 27, and thepermanent magnets may be replaced with permanent magnets of any knownconfiguration, such as V-shape, V- (concave-) shape with a base, bow-(reversed semicircular-) shape, or arrangement.

As explained in detail above, in the third through eighth embodiments,the deviation angle between the pole central line and the effectivepolar opening angle central line varies according to each magnetic pole.Therefore, the individual cogging torques generated by the respectivemagnetic poles are not in phase. Accordingly, a brushless DC motor whoseoverall cogging torque is reduced by the cancellation of the individualcogging torques is realized. In particular, by setting the deviationangle difference θ6 within the range of 0.2×θ7≦θ6≦θ5−(0.2×θ7) withrespect to the slot pitch angle θ5 and the slot opening θ7, the coggingtorque can be effectively reduced.

Moreover, considering the direction of the deviation angle, if threetypes of magnetic poles including a reference magnetic pole, a magneticpole having a deviation angle in the leading direction and a magneticpole having a deviation angle in the lagging direction are provided, itis also possible to cancel the cogging torques by three-phasecomposition. In this case, even if the individual cogging torques areasymmetrical waveforms or the like, it is possible to achieve aparticularly significant reduction in cogging torque. Further, in eitherof the two-phase and three-phase cases, by equalizing the number ofmagnetic poles having each deviation angle, more satisfactory resultsare obtained. In particular, in the three-phase case, by arranging allof the magnetic poles to have any one of the three deviation angles andequalizing the number of magnetic poles having each deviation angle, itis possible to reduce the overall cogging torque to near zeropractically.

Additionally, by arranging adjacent magnetic poles in thecircumferential direction not to have the same deviation angle, theinfluence of the deviation angle on the position of the center ofgravity of the rotor is reduced. It is thus possible to minimize thedeterioration of the rotary balance. In particular, in the three-phasecase, by arranging any three adjacent magnetic poles in thecircumferential direction to include all of the three deviation angles,the rotary balance can be almost perfectly maintained.

Besides, in the case where the rotor is a block construction in whichthe rotor is divided in the axial direction, if the blocks are arrangedso that the corresponding magnetic poles in the axial direction havemutually different deviation angles, there are merits on both thereduction in cogging torque and the maintenance of the rotary balance.

Furthermore, the reduction in cogging torque by the difference in thedeviation angle is achievable by providing convex portions correspondingto the magnetic poles on the outer circumference of the rotor andintroducing a deviation angle in the positions of the convex portions,instead of introducing a deviation angle in the effective polar openingangle central lines. In this case, the deviation in the positions of theconvex portions has the advantage that the influence on the rotarybalance is extremely small in comparison with the deviation in thepositions of the permanent magnets.

Note that in the third embodiment, etc., the magnetic poles having nodeviation between the pole central line and the effective polar openingangle central line and the magnetic pole having the deviation angle θ6are taken into consideration, but “magnetic poles having no deviation”are not essential. In short, the point is the presence of a relativedifference in the deviation angle between the magnetic poles. Thus, itis also possible to set a magnetic pole having a deviation angle θ0between the pole central line and the effective polar opening anglecentral line as a standard and provide magnetic poles having a deviationangle given by the addition of θ6 to θ0 or the subtraction of θ6 fromθ0. The same thing can also be said for the deviation angle of theconvex portions in the seventh or eighth embodiment. Note that acombination of the deviation angle of the convex portions in the seventhor eighth embodiment and the divided-block structure is of courseavailable.

(Ninth Embodiment)

FIG. 28 is a perspective view showing the construction of a stator of abrushless DC motor according to the ninth embodiment of the presentinvention. This stator 1 is formed by layering a number of thinelectromagnetic steel plates and fixing them integrally, and comprises ayoke 2 that is formed as an outer circumferential portion and teeth 3that are provided at equal intervals to protrude from the yoke 2 towardthe center. Adjacent teeth 3 form a slot 4 together with the yoke 2.Actually, armature windings (not shown) are wound on the teeth 3 andstored in the slots 4.

In the thin electromagnetic steel plate, a portion corresponding to thenotch portion 9 a is provided at the outer circumferential surfaceportion of the yoke 2 near the outside of a portion corresponding toevery third tooth 3. The portion corresponding to the notch portion 9 ahas a protrusion therein so as to facilitate layering and welding of thethin electromagnetic steel plates. Blocks are formed by layeringsubstantially an equal number of such thin electromagnetic steel platesat equal angle so that the notch portion 9 a of every third tooth 3 hassubstantially an equal length in the layering direction. These blocksare layered while displacing them at a predetermined angle in thecircumferential direction so that the portions corresponding to thenotch portions 9 a are aligned. The notch portions 9 a of each block areformed so that they do not overlap adjacent notch portions 9 a in thecircumferential direction.

Accordingly, as shown in the side view of the stator 1 of FIG. 29, thestator 1 is a construction comprising layers of a multilayer blocksegment having a notch portion 9 a on the outside of every third tooth 3and a thickness S11; a multilayer block segment which is displaced fromthe above multilayer block segment by an amount corresponding to onetooth 3 in the circumferential direction and has a notch portion 9 a onthe outside of every third tooth 3 and a thickness S21; a multilayerblock segment which is similarly displaced from the above multilayerblock segment by an amount corresponding to one tooth 3 in thecircumferential direction and has a notch portion 9 a on the outside ofevery third tooth 3 and a thickness S31; a multilayer block segmentwhich is similarly displaced from the above multilayer block segment byan amount corresponding to one tooth 3 in the circumferential directionand has a notch portion 9 a on the outside of every third tooth 3 and athickness S12; a multilayer block segment which is similarly displacedfrom the above multilayer block segment by an amount corresponding toone tooth 3 in the circumferential direction and has a notch portion 9 aon the outside of every third tooth 3 and a thickness S22; and amultilayer block segment which is similarly displaced from the abovemultilayer block segment by an amount corresponding to one tooth 3 inthe circumferential direction and has a notch portion 9 a on the outsideof every third tooth 3 and a thickness S32.

The multilayer block segments with the thickness S11, S21, S31 and themultilayer block segments with the thickness S12, S22, S32 have thenotch portions 9 a at the same positions in the circumferentialdirection respectively. Here, the following equation (4) is satisfied.S 11+S 12=S 21+S 22=S 31+S 32  (4)Besides, if the total thickness is S0, then the thickness of themultilayer block segments without the notch portions 9 a of each tooth 3is given by the following equation (5). $\begin{matrix}\begin{matrix}{{{S0} - \left( {{S11} + {S12}} \right)} = {{S0} - \left( {{S21} + {S22}} \right)}} \\{= {{S0} - \left( {{S31} + {S32}} \right)}}\end{matrix} & (5)\end{matrix}$

FIG. 30 is a view showing the state of magnetic flux in such a brushlessDC motor. Here, a rotor 5 constructed by attaching permanent magnets 7to the surface of a rotor core 6 is disposed in the stator 1 shown inFIG. 28. The stator 1 has the construction explained with reference toFIGS. 28 and 29. Note that the rotor 5. may be a buried-type rotorconstructed by burying the permanent magnets 7 in the rotor core 6.

In each portion of the stator 1, magnetic flux generated because of therelative positional relationship with each region between magnetic polesof the opposing permanent magnets 7 of the rotor 5 flows. The magneticflux is shown by indicating the flux amount of a magnetic path d at aposition where the notch portion 9 a whose length in the layeringdirection is equal to S11 is present on the outer circumference side ofthe tooth 3 of the stator 1 as φ4, indicating the flux amount of amagnetic path e which is adjacent to the magnetic path d and located ata position where the notch portion 9 a whose length in the layeringdirection is equal to S11 is present on the outer circumference side ofthe tooth 3 of the stator 1 as φ5, and indicating the flux amount of amagnetic path f adjacent to the magnetic path e as φ6.

Here, as shown in FIG. 30, if straight lines A, B and C are drawn fromthe center of the shaft hole of the rotor 5 toward the outercircumference of the stator 1 through the center of the slots 4, then,when a region between the magnetic poles of the permanent magnets 7 ofthe rotor 5 is positioned on the straight line A, the magnetic flux fromthe permanent magnets 7 near the region between the magnetic poles forma closed circuit of the flux amount φ4 by the magnetic path d shown by adotted line. Moreover, when the region between the magnetic poles of thepermanent magnets 7 of the rotor 5 reaches the straight line B as aresult of clockwise rotation of the rotor 5, the magnetic flux from thepermanent magnets 7 near the region between the magnetic poles form aclosed circuit of the flux amount φ5 by the magnetic path e shown by adotted line. When the region between the magnetic poles of the permanentmagnets 7 of the rotor 5 reaches the straight line C as a result offurther clockwise rotation of the rotor 5, the magnetic flux from thepermanent magnets 7 near the region between the magnetic poles form aclosed circuit of the flux amount φ6 by the magnetic path f shown by adotted line.

For example, suppose that the notch portions 9 a are aligned in thelayering direction over a multilayer block thickness S0 of the stator 1.At this time, the flux amount flowing in the yoke 2 having the notchportions 9 a in the layering direction of the teeth 3 is denoted as φ1a, and the flux amount flowing in the yoke 2 having no notch portions 9a in the layering direction of the teeth 3 is denoted as φ1 b. Then,both of portions having the notch portions 9 a and portions having nonotch portions 9 a in the layering direction are present in theconstruction of the stator 1 shown in FIG. 30. Therefore, when theregion between the magnetic poles of the permanent magnets 7 of therotor 5 is positioned on the straight line A, the flux amount φ4 of themagnetic path d with the straight line A as the center is given by thefollowing equation (6).φ4=φ1 a×(S 11+S 12)/S 0+φ1 b×(S 0−(S 11+S 12))/S 0  (6)

Similarly, when the region between the magnetic poles of the permanentmagnets 7 of the rotor 5 is positioned on the straight line B, the fluxamount φ5 of the magnetic path e with the straight line B as the centeris given by the following equation (7).φ5=φ1 a×(S 21+S 22)/S 0+φ1 b×(S 0−(S 21+S 22))/S 0  (7)

Likewise, when the region between the magnetic poles of the permanentmagnets 7 of the rotor 5 is positioned on the straight line C, the fluxamount φ6 of the magnetic path f with the straight line C as the centeris given by the following equation (8).φ6=φ1 a×(S 31+S 32)/S 0+φ1 b×(S 0−(S 31+S 32))/S 0  (8)

Here, it is apparent by substituting equations (4) and (5) for equations(6), (7) and (8) that the magnetic coupling between the stator 1 and therotor 5 can be made stable coupling with less fluctuation because theflux amounts φ4, φ5 and φ6 of the magnetic paths d, e and f in the yoke2 are constant even when the region between the magnetic poles of therotor 5 is positioned on any one of the straight lines A, B and C. Inother words, the cogging torque can never increase locally depending onthe rotational position of the rotor 5, thereby restricting thegeneration of sound and vibration resulting from the cogging torque.

Regarding the magnitude of the cogging toque, as shown in FIG. 31, sincethere is no protruding portion on the straight lines A to C, the torquedoes not change abruptly and thereby reducing the cogging torquerelatively. In FIG. 31, the vertical axis indicates the cogging torqueTC, while the horizontal axis indicates the rotation angle θ of therotor 5, and the positions of the straight lines A to C in FIG. 30correspond to the positions of the straight lines A to C in FIG. 31.

It is also apparent from the above description and equations (4) to (8)that the same effects are obtained even when the notch portions 9 a onthe outer circumference side of the stator 1 of an arbitrary tooth 3 aredistributed to any positions in the layering direction. For example, inthe construction shown in FIG. 30, if the thin electromagnetic steelplates are layered by introducing a displacement corresponding to up toone tooth 3 whenever one thin electromagnetic steel plate is layered,the notch portions 9 a are aligned whenever three thin electromagneticsteel plates are layered. Accordingly, if a large number of plates arelayered, the total length, in the layering direction, of the notchportions 9 a provided for each tooth 3 of the stator 1 becomessubstantially equal, and thus the objective of the ninth embodiment canbe achieved.

Besides, for example, in the method of layering the thin electromagneticsteel plates of the stator 1 by introducing a displacement of apredetermined angle whenever one thin electromagnetic steel plate islayered as described above, the known automatic clamp control forpunching and integrally fixing the thin electromagnetic steel plates atthe same time becomes complicated, and the punching speed can not beincreased. However, by employing a block structure including a pluralityof notch portions 9 a aligned in the layering direction, it is possibleto simplify the punching control and increase the punching speed,thereby improving the productivity.

Further, in the above-described example, for the thin electromagneticplates that forms the stator 1, a portion corresponding to the notchportion 9 a is provided at an outer circumferential portion equivalentto every third tooth 3. However, as shown in FIG. 32, for the thinelectromagnetic plates forming the stator 1, a portion corresponding tothe notch portion 9 b may be provided at an outer circumferentialportion corresponding to every other tooth 3. In this case, even whenthe notch portion 9 b is provided in the magnetic path, the regionbetween the magnetic poles of the permanent magnets 7 of the rotor 5 ispresent to face any one of the slots 4, thereby achieving the objectiveof the ninth embodiment.

If the thin electromagnetic steel plates are layered while displacingthem at a predetermined angle so that the notch portion 9 b is providedin the outer circumferential portion of every other tooth 3, it ispossible to punch every notch portion 9 b at a minimum displacementangle corresponding to a single tooth 3, and thus the number of punchingprocesses is reduced. For example, in the case where the notch portions9 b are punched by moving the stator 1 at a desired angle, it ispossible to increase the punching speed and improve the productivity. Inother case, if the total length, in the layering direction, of the notchportions 9 b provided for each tooth 3 of the stator 1 is substantiallyequal, the objective of the ninth embodiment can be achieved.

In particular, in the case where a countermeasure against deteriorationof cogging torque is implemented by a slight adjustment of coggingtorque, the notch portions 9 a to be provided on the outer circumferenceside of the stator 1 are arranged within the pitch range of the teeth 3so that they do not overlap each other in the circumferential directionof the stator 1. In other words, it is necessary that adjacent notchportions 9 a in the layering direction do not overlap each other.

Here, as shown in FIG. 33, notch portions 9 c and 9 d are provided onthe outer circumference side of the teeth 3, the pitch angle of theteeth 3 is made Bp, the central angle corresponding to the outercircumferential portion of each of the notch portions 9 c and 9 d ismade Bk, the length of the notch portion 9 c on the upper side of thelayer is made S11, the length of the notch portion 9 d on the lower sideof the layer is made S21, an outer circumferential surface where nonotch portions 9 c and 9 d are present is made a circular-arc portion 40to obtain the layered condition as shown in FIGS. 28 and 29.

When Bp <Bk, i.e., when adjacent notch portion 9 c and notch portion 9 din the circumferential direction overlap each other, since both ends ofthis overlapped portion protrude, the magnetic flux easily leaks fromthe tips of these ends. The amount of this leakage magnetic flux changeslargely depending on a slight difference in the configuration of thenotch portions, and the magnetic flux intended to pass through themagnetic path of the yoke 2 of the stator 1 leaks outside. Thus, theflux amount passing through the yoke 2 in the outer circumferentialportion of the teeth 3 is not stable and varies according to the notchwidth of each of the notch portion 9 c and notch portion 9 d, resultingin deterioration of cogging torque. In particular, this phenomenonappears more noticeably as the notch portions 9 c, 9 d and thecircular-arc portion 40 form a more acute angle. Moreover, a small-sizehigh-output motor with higher magnetic flux density in the yoke 2 islargely affected by this phenomenon.

Accordingly, in FIG. 33, the central angle Bk corresponding to the outercircumferential portion of each of the notch portions 9 c and 9 d is setso as not to be larger than at least the pitch Bp of the teeth 3, andthe notch portion 9 c and the circular-arc portion 40 of the outercircumferential surface are alternately provided on the outercircumference side of the stator 1. Thus, it is seen from FIGS. 33, 28and 29 that the portions corresponding to the notch portions 9 c of thelayered thin electromagnetic steel plates are arranged so as not tooverlap the portions corresponding to adjacent notch portions 9 d of thethin electromagnetic steel plates displaced at an angle in the crosssection in the layering direction.

When the central angle Bk is increased to a maximum, Bp=Bk. In thiscase, on the outer circumference of the stator 1 in the side view, anend of the notch portion 9 c and an end of the notch portion 9 d arearranged in contact with each other in the circumferential direction.Accordingly, there is no protrudent portion where the tips of theabove-mentioned two ends are in contact with each other and the magneticflux leaks, and thus the leakage magnetic flux from the stator 1 to theoutside can be significantly reduced.

Note that adjacent notch portions in the layering direction have beenexplained with reference to the notch portions 9 c and 9 d on the outercircumference of the stator 1, but the same explanation is applied toall the notch portions shown in FIGS. 28 and 29. Moreover, by settingBp=Bk, in the outer circumference of the stator 1, the portions of thecorner sections where burr is created due to the notch portions 9 c and9 d can be reduced to one second, thereby decreasing fitting defects infitting into a case or the like.

Note that while the above-described ninth embodiment illustrates thenotch portions of the outer circumference of the stator, the cavityportions provided in the outer circumference side of the stator can beexplained in the same manner.

1-4. (Canceled)
 5. A brushless DC motor comprising: a rotor havingplural magnetic poles provided at an equal pitch in a circumferentialdirection by mounting permanent magnets in magnet mounting holes; and astator having plural slots arranged at an equal pitch in acircumferential direction, wherein the magnetic poles of said rotorinclude magnetic poles whose magnet deviation angle formed by a centralline of an effective polar opening angle and a central line of saidmagnet mounting hole is a first angle; and magnetic poles whose magnetdeviation angle is a second angle different from the first angle.
 6. Thebrushless DC motor of claim 5, p1 a difference θ6 between said secondangle and said first angle is within a range defined by0.2×θ7≦θ6≦θ5−(0.2×θ7)where θ5 is a slot pitch angle of said stator, andθ7 is a slot opening angle of said stator.
 7. The brushless DC motor ofclaim 5, wherein the number of the magnetic poles whose magnet deviationangle is the first angle and the number of the magnetic poles whosemagnet deviation angle is the second angle are equal to each other. 8.The brushless DC motor of claim 5, wherein the magnetic pole whosemagnet deviation angle is the first angle and the magnetic pole whosemagnet deviation angle is the second angle are arranged next to eachother on said rotor.
 9. The brushless DC motor of claim 5, wherein saidrotor is divided into plural bocks in a rotation axis direction, and themagnetic pole whose magnet deviation angle is the first angle and themagnetic pole whose magnet deviation angle is the second angle arearranged at corresponding positions in the rotation axis direction indifferent blocks.
 10. The brushless DC motor of claim 6, wherein themagnetic poles of said rotor further include magnetic poles whose magnetdeviation angle is a third angle different from said first and secondangles, and a difference between the third angle and the first angle hasthe same value as and opposite sign to a difference between the secondangle and the first angle.
 11. The brushless DC motor of claim 10,wherein the number of the magnetic poles whose magnet deviation angle isthe first angle, the number of the magnetic poles whose magnet deviationangle is the second angle and the number of the magnetic poles whosemagnet deviation angle is the third angle are equal to each other. 12.The brushless DC motor of claim 11, wherein a total number of themagnetic poles of said rotor is an integral multiple of 6, and all ofthe magnetic poles of said rotor are any magnetic pole among themagnetic poles whose magnet deviation angle is the first angle, themagnetic poles whose magnet deviation angle is the second angle and themagnetic poles whose magnet deviation angle is the third angle.
 13. Abrushless DC motor comprising: a rotor having plural magnetic polesprovided at an equal pitch in a circumferential direction by mountingpermanent magnets in magnet mounting holes; and a stator having pluralslots arranged at an equal pitch in a circumferential direction, whereinsaid rotor comprises convex portions corresponding to the magnetic poleson its circumference, and the magnetic poles of said rotor includemagnetic poles whose convex portion deviation angle formed by a centralline of said convex portion and a central line of said magnet mountinghole is a first angle; and magnetic poles whose convex portion deviationangle is a second angle different from the first angle.
 14. A brushlessDC motor comprising: a rotor having plural magnetic poles provided at anequal pitch in a circumferential direction by mounting permanent magnetsin magnet mounting holes; and a stator having plural slots arranged atan equal pitch in a circumferential direction, wherein said rotorcomprises convex portions corresponding to the magnetic poles on itscircumference, and the magnetic poles of said rotor include magneticpoles whose magnet deviation angle formed by a central line of aneffective polar opening angle and the central line of said magnetmounting hole and whose convex portion deviation angle formed by acentral line of said convex portion and a central line of said magnetmounting hole are both first angle; and magnetic poles whose magnetdeviation angle and convex portion deviation angle are both second angledifferent from the first angle. 15-19 (canceled)